How feasible would it be to have a nice "xxx kb left" green bar at the bottom area of NESmaker? That would really help us to manage memory in our game. Should I try to use shorter songs? Or maybe reuse this sprite to make this one? Etc.

The bar would hopefully be refreshed instantly, or at least refreshed after compiling the game. When the project is completely new, the bar would be green, and it would get closer and closer to red as you add stuff to your game.

This was always a plan. It won't be implemented in initial release, but I can imagine it being an add on!

It's tough because the program really isn't keeping real time track of that stuff...until it renders, but if there is too much data at that point, it just gives you an error and doesn't compile, meaning it can't tell you the memory that's left. However, I can tell you the error you'd likely get if one of the banks overflowed for now.

Yes, I understand NESmaker is going to get better and better with time, with more and more functionalities. There are so many things you have to tackle in order to make the experience user-friendly and seamless, especially in the case of the NES.

I guess a more rudimentary "check memory left" button that would compile and render in the background in order to display the memory left next to the button would be a great start.